The present invention relates to sensors, and more particularly to sensors with enhanced response.
Electrochemical sensors, and especially amperometric sensors are described, for example, in published European Patent Specifications 78636, 125137, and 184895, among others. Such sensors are based on enzyme systems, optionally with electron transfer agents to assist charge transfer, and can be used to detect, for example, naturally occurring metabolites such as glucose or cholesterol, or substances introduced to the body such as alcohol or a drug. In a typical mode of use, the system is set up so that there is a response at the amperometric electrochemical sensor which is dependent on the concentration of analyte.
The amperometric sensor can be constructed as a strip sensor, as described for instance in EP 127958. For preference, the strip element is a dry strip disposable test element, for use in conjunction with a read-out device to which the test element can be connected to give a reading of the analyte level after application of a sample to a target area of the test element. For example, the read-out device can be a hand-held or desk-top instrument.
After a predetermined incubation period to allow for reaction, a reading can be taken by poising the working electrode at a suitable voltage, and monitoring the resultant current for a set time. By reference to currents generated under standardised conditions with known concentrations of analyte, the current can be used to give a quantitative value of the analyte level.
Such amperometric strip sensors can be used to give a disposable strip sensor for glucose, as illustrated by the "ExacTech" (Trade Mark) sensor which is commercially available.
A mediator is not always necessary for an amperometric strip sensor. For example, EP 184895 describes a system for monitoring paracetamol. An electrode poised at a suitable potential is contacted with a sample suspected of containing an N-acylated primary aromatic amine, typically paracetamol, and with an enzyme capable of catalysing the hydrolysis of the N-acylated primary aromatic amine. The current flowing in the electrode is a measure of the quantity of hydrolysis products formed and thereby of the concentration of N-acylated primary aromatic amine in the sample.
In practice with the sensors of EP 184895, it is sometimes difficult to achieve reliable results when using blood samples from patients receiving medication for paracetamol overdose. This difficulty particularly arises, for instance, with patients receiving antidote treatment. The presence of the antidote interferes with the assay reaction.
More generally, with some enzyme systems which might be used in such an amperometric sensor, there is a problem that the product derived from the substrate by enzyme activity is an inhibitor of the enzyme. When the product inhibits the enzyme, there is reduced enzyme activity, leading to a reduction in the charge transfer which might otherwise be obtained.